Assessing the utility of Fe/Al and Fe-speciation to record water column redox conditions in carbonate-rich sediments

Geochemical proxies based on Fe abundance (Fe/Al) and Fe-speciation have been widely applied to marine sediments in order to unravel paleo-depositional redox conditions though geological time. To date, however, these Fe proxies have only been calibrated in relation to modern and ancient fine-grained siliciclastic marine sediments. This clearly limits their use, particularly in relation to carbonate-rich sediments and rocks. To address this, we here explore the applicability of Fe-based redox proxies in carbonates through three approaches. First, we have compiled Fe/Al data for modern marine sediments to investigate variability in Fe-enrichments as a function of carbonate content and depositional setting. Second, we have expanded this approach with a compilation of new and existing Fe-speciation data for modern and ancient marine sediments deposited under oxic and euxinic (anoxic sulfidic) water column conditions. Finally, we show new data from paired limestone and dolomite sample sets to demonstrate the potential significance of deep burial dolomitization on the Fe-speciation redox proxy. Modern marine sediments deposited under oxic conditions show no relationship between Fe/Al and carbonate content. These sediments have an average Fe/Al ratio of 0.55±0.11, with some higher values potentially being attributable to steady-state early diagenetic remobilization of Fe towards the sediment-water interface. In contrast, significant Fe/Al enrichments occur as a consequence of water column Fe mineral formation and deposition, either under anoxic conditions, or due to input of anoxic hydrothermal fluids into oxic seawater. Iron speciation data also show no direct correlation with carbonate content, and instead three groups can be distinguished based on total Fe (Fe) and organic C contents. Sediments deposited under oxic water column conditions, with Fe >0.5wt.%, generally plot below the lower Fe/Fe siliciclastic reference threshold (0.38) for distinguishing anoxic environments, regardless of carbonate content. Also consistent with siliciclastic calibrations, carbonate-rich sediments deposited under anoxic water column conditions tend to have Fe/Fe ratios >0.38. In contrast, oxic carbonate-rich sediments with low Fe (<0.5wt.%) and low organic C (<0.5wt.%) routinely give spuriously high Fe/Fe ratios, suggesting that the use of Fe-speciation for such samples is not appropriate for evaluating water column redox conditions. Taken together, our data suggest that an Fe content of >0.5wt.%, rather than carbonate content, represents the most suitable basis for identifying whether carbonate-rich sediments are appropriate for Fe-based redox proxy analyses. Analysis of burial dolostones suggests that the Fe-speciation proxy may also be compromised by deep burial dolomitization, where there has been a source of mobile Fe to enrich rocks during recrystallization. However, pre-screening can identify such samples. This new assessment expands the utility of Fe-based redox proxies to also incorporate appropriate carbonate-rich rocks, provided that care is taken to assess the possible impact of deep burial dolomitization.